Flat Roof Coverings: EPDM, GRP and Felt

EPDM rubber membrane

A single sheet of synthetic rubber bonded to the deck, so a typical domestic roof has no joints in the field at all. It stays flexible in cold weather and moves with the building rather than resisting it.

Its weakness is mechanical: it can be punctured by dropped tools or sharp debris, so it is less suited to a roof that will be walked on regularly without protection.

GRP fibreglass

Resin and matting laid over a specific board deck and cured in place, producing a hard, seamless, walkable surface with neat trims. It suits balconies and roofs with a lot of penetrations because detailing can be formed around anything.

Hard-wearing and walkable without extra protection · Application is weather-dependent and needs dry conditions to cure · Rigid, so movement in the deck can cause cracking at stress points · Deck build-up and board type must follow the system specification

Torch-on built-up felt

Modern high-performance felt is applied in two or three bonded layers and bears little relation to the single-layer felt roofs it is remembered for. It is economical, repairable and tolerant of imperfect decks.

The trade-off is joints, and joints are where felt roofs eventually fail, particularly at upstands and around penetrations.

What actually determines lifespan

The covering matters less than the things underneath and around it: adequate falls so water leaves the roof, a properly ventilated or correctly specified warm-roof build-up, upstands high enough at abutments, and outlets sized and positioned so they do not block.

A ponding roof will defeat any covering. If water is standing on the roof a day after rain, the falls need correcting before recovering it.

Frequently asked questions

Which flat roof covering lasts longest?

Well-installed EPDM and GRP both give long service lives, and modern built-up felt is not far behind. Installation quality and roof falls make more difference than the choice of material.

Can you walk on a flat roof?

GRP is the most tolerant of foot traffic. EPDM and felt can be walked on for maintenance but benefit from walkway protection where access is regular.

Warm roof or cold roof?

Warm roof construction — insulation above the deck — is now the normal specification because it avoids the ventilation problems and interstitial condensation risk of cold roofs.

Can a flat roof be recovered over the existing one?

Sometimes, if the deck is dry and sound and the build-up allows the extra thickness. Where the insulation is wet, overlaying seals the moisture in and the deck rots underneath.